Fundamental performance requirments for a photographic silver halide emulsion include high sensitivity, substantially no fogging and fine graininess.
In order to enhance the sensitivity of the emulsion, it is necessary to (1) increase the number of photons absorbed by each grain; (2) efficiently convert photoelectrons released by light absorption into a silver cluster (latent image); and (3) enhance development activity to effectively utilize latent image formation. The number of photons absorbed by a single grain can be increased by enlarging the grain size, but image quality is lowered. The enhancement of development activity is an effective means for enhancing sensitivity, but graininess is generally deteriorated when development is such a parallet type as color development. In order to enhance sensitivity without deleterious effect on graininess, it is most preferred to enhance the efficiency for converting photoelectrons into a latent image, namely, to increase the quantum sensitivity. It is necessary to remove inefficient steps such as recombination (recombination of an electron e.crclbar. with a hole.sym.) and latent image dispersion as much as possible to increase the quantum sensitivity. It is known that a reduction sensitization method wherein fine silver nuclei having no development activity are formed in the interior of the silver halide grain or on the surface thereof, is effective in the prevention of recombination.
It is disclosed in Photogr. Sci. Eng., 19, 49 (1975) that when reduction sensitization is carried out such that the coated film of a gold and sulfur sensitized emulsion is degassed in vacuo and then heat-treated in a hydrogen gas atmosphere, sensitivity is increased to such an extent that the degree of fogging is low in comparison with a conventional reduction sensitization method. The sensitization method is well known as hydrogen sensitization and is effective as a sensitivity-increasing means on a laboratory scale. Furthermore, hydrogen sensitization is practically carried out in astronomical applications for the photographing of stars.
Reduction sensitization methods have long been proposed. For example, tin compounds have been disclosed as being useful reduction sensitizing agents in U.S. Pat. No. 2,487,850 to Carroll, polyamine compounds for this purpose are disclosed in U.S. Pat. No. 2,512,925 to Lowe et al., and thiourea dioxide compounds are disclosed in U.K. Patent 789,823 to Fallens. Comparisons of the properties of silver nuclei prepared by various reduction sensitization methods are disclosed in Photographic science and Engineering, Vol. 23, page 113 (1979), by Collier. Collier employed methods using dimethylamine borane, stannous chloride, hydrazines, ripening at a high pH and ripening at a low pAg. Furthermore, reduction sensitization methods are disclosed in U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777 and 3,930,867. Selection criteria for reduction sensitizing agents as well as methods for using the same are disclosed in JP-B-57-33572 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-58-1410 and JP-A-57-179835 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Methods for improving the preservability of reduction sensitized emulsions are described in JP-A-57-82831 and JP-A-60-178445. As described above, many proposals for reduction sensitization have been set forth. However, the increase in sensitivity using these reduction sensitization techniques is still inadequate in comparison with hydrogen sensitization wherein photographic materials are treated with hydrogen gas in vacuo, as reported by Moisar et al., Journal of Imaging Science, Vol. 29, page 233 (1985).
Conventional reduction sensitization methods are inadequate for state of the art photographic materials requiring high sensitivity and high image quality. Furthermore, hydrogen sensitization is disadvantageous in that the sensitization effect is lost when photographic materials thus prepared are exposed to air. Accordingly, the hydrogen sensitization method is unsuitable for many photographic applications.